(1) Field of the Invention
The present invention relates to an image forming apparatus which has a means of measuring the toner density and correcting its measurement to a correct value and can be applied to electrophotographic copiers, laser beam printers, etc.
(2) Description of the Prior Art
In image forming apparatuses using an electrophotographic process such as copiers, laser beam printers, etc., the photoreceptor which has been charged is exposed to light to create a static latent image, and the latent image is developed into a toner image whilst a developing bias voltage is being applied. The image forming apparatus of this kind has suffered from an image degradation problem in that the density of the toner image fluctuates due to change in surrounding conditions such as temperature, humidity etc., or due to change with the passage of time, i.e., aging, so that medium-gradation image density is not reproduced uniformly. In order to avoid image degradation, various methods have been proposed. One of these methods comprises the steps of forming a toner image under prescribed conditions on the photoreceptor or a transfer member which is arranged in contact with the photoreceptor; reading the intensity of reflected light using a reflection type optical sensor; and controlling the control parameters for image forming based on its difference from a reference value. For example, Japanese Patent Application Laid-Open Sho 60-260066 discloses a configuration in which two toner images having different densities, i.e., dark and light are formed on the photoreceptor so that the densities are read by two reflection type sensors.
In Japanese Patent Application Laid-Open Sho 60-260066, it is specified that the two sensors preferably have equivalent characteristics. In practice, however, the characteristics of the light emitter and photosensor of such a sensor differs from one to another due to their individuality and varies due to temperature, degradation from a prolonged period of its usage and its geometry, i.e., the angle, distance etc. between the sensor and the target, producing the problem of degraded accuracy of measurement.
It is possible to achieve a relatively exact measurement of two levels of density if the operational points of the sensors for the two levels of density are corrected. However, when three or more colors of toner are used or the sensor has non-linear characteristics, it is not always possible to make exact correction for all the colors of toner.
In the case where toner images for the measurement of density are formed under the specified conditions and based on the result of the measurement of density the image forming conditions for the image forming system will be corrected in order to stabilize the operation of forming toner images, the light intensity for measurement is set up in conformity with the density of the toner image in the initial stage. Even in such a configuration, however, the toner image for the measurement of density will vary due to change in surrounding conditions or due to the passage of time, and therefore problems often occur that the toner density does not go with the light intensity for measurement.